Due to their non-emissive nature, traditional liquid crystal Flat Panel Displays ("FPD") use either reflected light or backlights (e.g. cold cathode or hot cathode fluorescent tubes) as white light sources. When a backlight is employed, the white light generated by the backlight is directed to a matrix consisting of individual liquid crystal pixels. Light entering each individual pixel of the matrix is either blocked or transmitted, depending on whether or not a sufficient electric field has been applied to that individual pixel.
In an active matrix type liquid crystal display ("LCD"), each liquid crystal pixel is directly addressable (i.e. able to be directly switched between the "on" and "off" modes by the application of an electric field). Such addressing is typically accomplished with the aid of thin film transistors ("TFT"). In color active matrix LCDs, as well as in passive matrix color LCDs, individual liquid crystal pixels are grouped into formations of several sub-pixels. Within the formation, each individual sub-pixel is associated with a color filter (e.g., in a three sub-pixel formation, each sub-pixel would be associated with either a red, green or blue filter). Typical formations include three or four sub-pixels arranged in a vertical stripe, quad, triad or horizontal stripe configuration, although other configurations and filter colors are possible.
Details regarding conventional LCD configurations, conventional backlights, and the operation of conventional liquid crystal displays are well known in the art. See, for example, S. W. Depp and W. E. Howard "Flat Panel Displays" Scientific American, page 90, (March, 1993); H. Miller, "An Examination of Active Matrix Technologies and Components", Sharp LCD Application Note, pages 2-10-2-14; and A. Dragon, "Backlighting," Sharp Application Notes, pages 2-100-2-106.
FIG. 1 illustrates a simplified cross sectional representation of a generic conventional backlit color LCD 10 having liquid crystal sub-pixels 12, 14, 16, and 18, i.e. two red sub-pixels, 12 and 16, and two green sub-pixels, 14 and 18. The white light source, used as a backlight in the LCD configuration of FIG. 1, is a conventional cold cathode fluorescent tube 20. Associated with this light source are light guide 22, diffuser 24 and brightness enhancement filter 26. The entire arrangement, 28, of all liquid crystal pixels and sub-pixels, is known as a liquid crystal matrix. (The liquid crystal matrix of a typical LCD can contain on the order of several hundred thousand pixels and several times more sub-pixels. For clarity, only four sub-pixels are shown in FIG. 1). Each liquid crystal sub-pixel (12, 14, 16 and 18) has a first polarizer 30 on first glass substrate 32, a TFT driven liquid crystal cell 34, a color filter 36, second glass substrate 38 and second polarizer 40. In sub-pixels 12 and 16, color filter 36 is a red filter and in pixels 14 and 18 a green filter.
During operation of the generic LCD 10 of FIG. 1, white light leaving brightness enhancement filter 26 travels to the each of the individual liquid crystal sub-pixels 12, 14, 16 and 18. Depending upon the magnitude of an electric field applied to each liquid crystal sub-pixel (by means not shown), the white light incident on first polarizer 30 is either (i) transmitted through the entire liquid crystal sub-pixel, including color filter 36 and second polarizer 40, and therefore exits polarizer 40 as colored light or (ii) blocked by operation of the liquid crystal sub-pixel. In order to block the incident light, the polarization of the light which exits first polarizer 30, through first glass substrate 32, is rotated by liquid crystal molecules (not shown) contained in liquid crystal cell 34 such that the colored light leaving color filter 36 is blocked by second polarizer 40.
A major disadvantage of a conventional backlit color LCD is that a majority of the light generated by the white light source is lost due to the less than ideal transparency associated with each of the liquid crystal display components. For example, color filters employed in LCDs typically have a light transmission efficiency (defined as the percentage of incident light that is transmitted through the filter) of between 20 and 33%. See, P. Pleshko, "Overview and Status of Information Displays" Society for Information Display, 1992 Seminar Lecture Notes, May 18. Therefore, if a red-filtered sub-pixel is in the "on" (light transmitting) mode, at least 66% of the white light incident on the red-filtered liquid crystal sub-pixel (i.e. the non-red wavelength portions of the white light) is blocked by the filter and therefore wasted. Likewise, the green and blue-filtered liquid crystal sub-pixels, even when transmitting light, waste at least 66% of the incident white light. Moreover, as illustrated by the typical values in Table 1, the overall efficiency of the light transmission, taking into account the efficiency of each of the liquid crystal display components, is typically only around 3-4% (see, Pleshko, supra at page M-0/63).
TABLE 1 ______________________________________ Conventional Backlit Color LCD Cumulative Component Transmissivity Transmissivity ______________________________________ Backlight Components 0.56 0.56 Polarizer 0.8 0.4 Substrate 0.945 0.37 Color Filters 0.2 0.075 Liquid Crystal 0.65 0.049 Substrate 0.945 0.0464 Polarizer 0.80 0.037 Total 0.037 ______________________________________
Other disadvantages associated with conventional backlit Liquid crystal displays are the relatively high cost of the color filters and an inability to generate a high intensity image at a relatively low power input (typically measured in units of lumens per watt).
One proposed alternative to conventional cold cathode or hot cathode fluorescent tubes backlights is a cathodoluminescent lamp employing thin film edge emitting devices as electron sources. See Akinwande, et al., "Thin Film Edge Emitter Vacuum Microelectronics Devices for Lamp/Backlight Applications," Eighth International Vacuum Microelectronics Conference Technical Digest, Jul. 30-Aug. 3, 1995, page 418. This configuration, however, still requires color filters, with their attendant inefficiencies, to produce a multi-color image.
As a result of the foregoing, what is still needed in the art is a cost effective, low power, multi-color liquid crystal display with a high light transmission efficiency and brightness.